CHAPTER 6: TIME DOMAIN ELECTROMAGNETIC PROSPECTING METHODS
Misac N. Nabighian, James C. Macnae, 1991. "TIME DOMAIN ELECTROMAGNETIC PROSPECTING METHODS", Electromagnetic Methods in Applied Geophysics: Volume 2, Application, Parts A and B, Misac N. Nabighian
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In the traditional frequency-domain electromagnetic (FEM) methods of exploration the ground is energized by passing an alternating current (ac) through an ungrounded loop situated usually on or above the surface of the earth. The primary field of the loop will induce eddy currents in all conductors present in the earth. The secondary electromagnetic (EM) fields due to these induced currents, together with the primary EM field, are recorded with a suitable receiver at various points in space. In general, the secondary EM field at the receiver, which contains all the information regarding the underground conductors, may be several orders of magnitude smaller than the primary field. Under these conditions the separation of the measured total EM field into its primary and secondary parts is difficult.